Cooling towers are the most common means used for recovering water which has been used to refrigerate some thermal process equipment or thermal machine. In a typical arrangement of a conventional cooling tower, a heat load is transferred to the water circulating through the heat exchanger and piped unto the cooling tower. How water is sprayed through nozzles on to the fill which includes a splash deck or laminar materials; the object is to break-up the mass of water into the largest possible surface; this surface is the actual heat transfer surface which brings the water into contact with the flowing air. Air circulation is usually obtained with fans of various types.
As the hot water and the air come into contact with each other, there is a transfer of heat from one to the other, but simultaneously there is also a transfer of mass, since a fraction of the water has vaporized and has been carried away by the moving air. The heat exchanging process is termed "evaporative", because the sensible heat of the water is converted into latent heat as it generates "vapor". Thus, this evaporation causes a constant loss of water in the system. Typically, the amount of water lost as vapor is replenished in the basin via a float valve. When the make-up water contains solids in solution, such as calcium carbonate, sulfates, magnesium, etc., the water is said to be "hard". As explained earlier, the cooling process is evaporative, the water vapor which leaves the cooling tower does not carry the solids it originally contained in its liquid state. These solids which remain in the mass of circulating water raise the overall content of solids until the water reaches a point of maximum saturation and any excess shall precipitate. These solids adhere on hot surfaces of the heat exchangers; successive layers will form a crust of scale with its detrimental effects on the overall heat transfer coefficients. A way to reduce the formation of scale is to increase the amount of volume of make-up water into the basin. This excess of make-up water dilutes the concentration of the solids contained in the open circuit water. The primary function of any cooling tower, conventional or closed circuit, is to economize water, thus, there is an economic limit to the use of the continuous bleed-off. There are cities where the authorities charge very high fines for this waste of water plus a penalty for the excessive use of the city sewage system.
A great part of the scale can be removed by chemical or mechanical means, however, the thin residue which usually remains over the cleaned surface enhances the rapid formation of new layers of scale; the process can repeat itself only a few timees, and a time comes when the thermal insulation of the scale is of such a magnitude that the heat exchanger must be scrapped and replaced by a new one. Of course, during all this time, the output or performance of the equipment served by the fouled heat exchanger has diminished; the scale would then be responsible for the drop in production, severed profits, lack of refrigeration, increase of the energy bills, break-down of furnaces, etc., etc. these uneconomical effects were noticed many years ago by the manufacturers of cooling tower; in response they created Closed Circuit Cooling Towers or Evaporative Fluid Coolers. A typical design of this equipment includes hot water flowing through a pipe connected to a bank of coils which is placed inside the cooling tower. The hot water flowing in the inside of the heat exchanger is indirectly cooled by the water that wets the outside surface of the tube bank. The cold water of the closed circuit is pumped back to the heat load by means of a pump. An expansion tank is required. The hot water keeps recirculating through the heat load and the cooling coils without ever coming into contact with the air; all the heat removed from the water of this closed system is sensible heat, thus no mass is lost or transferred by evaporation. The heat surrendered by the fluid inside of the coils is picked up by the water flowing over the outside of the tube; this water, the open circuit, comes into contact with the air. The heat transfer process between the open circuit water and the air is "evaporative" and identical to the process described for the conventional cooling tower described above.
The evaporative loss in the open circuit is compensated by the float valve. Any "hardness" in the make-up water shall now have an effect only on the outer surface of the heat exchanger. In summary, the Closed Circuit Evaporative Cooler has not eliminated the problem of scale build-up, it has simply changed the location of its effects; instead of fouling the surfaces of the heat load exchanger, the scale build-up takes place on the outside of the tubes or coils that comprise the Closed Circuit Heat exchanger inside the tower.
No doubt it's easier to attack and get rid of the scale deposited on the tower heat exchanger however, the cleaning is never absolute and gradually the efficiency drops producing the same detrimental effects described for conventional cooling tower. In my U.S. Pat. No. 4,443,389 and my Argentinian Patent Nos. 195,525 of Oct. 15, 1973 and 206,846 of Aug. 23, 1976, I disclosed a novel efficient and useful helical coil heat exchange structure and system for significantly reducing scale build-up.
Removing scale by chemical means is the usual manner. A reduction of the pH of the water in the open circuit is the prime reason to use chemicals and acid solutions. The greater the acidity, the better the descaling of the tube surfaces; however, as most tube banks are steel or other metals, the acids favor the descaling but then, the metal can be gradually destroyed.
This destruction does not occur when the tube banks are made of platic materials. In recent years a few manufacturers of Closed Evaporative Coolers have developed plastic heat exchangers. The commercial results (see Cooling Tower Institute Paper TP-261-A "A New Type of Closed Circuit Cooling Tower with Plastic Heat exchangers", Jan. 31, 1983) have been excellent and several points have been proven:
(a) The plastic surface tends to reject the adherence of scale.
(b) Chemicals used, either to remove any scale on the tubes or to destroy algae or other organics in the open circuit water, are harmless to the plastic materials used.
(c) The overall coefficient of heat transfer, as compared with all metal tube banks, is not as bad as it may look.
However, as shown in the above CTI paper, the plastic heat exchanger must have 3.5 to 4 times more transfer surface.
The object of the invention is to provide an improved heat exchanger. A further object of the invention is to provide an improved plastic heat exchanger. Another object of the present invention is to provide an improved plastic heat exchanger. Another object of the invention is to reduce heat transfer distance between the latent heat bearing fluid and the heat exchange fluid by making the interface member in thin inert plastic film, as thin as possible while retaining sufficient material to assure long life against abrasion by the flow of fluid material. Still another object of the invention is to provide plastic film heat exchange with significantly improved operating efficiency, reduced weight and overall physical size.
According to the present invention, a very thin plastic structure serves as the basic medium of heat exchange interface between the latent heat bearing first fluid on the one hand and the heat receptive second fluid on the other hand, and a rigidifying structure substantially parallel to the flow path of the first fluid. The plastic, preferably in the form of tubes, is very flabby and the shape of the tube, circular, for example, is only attained when the tube is submitted to a predetermined pressure, which is preferably low.